Re: [ovs-dev] [PATCH net v2] openvswitch: meter: fix race when getting now_ms.

[Ilya Maximets <i.maximets@ovn.org>]

On 5/13/21 3:08 PM, Tao Liu wrote:
> We have observed meters working unexpected if traffic is 3+Gbit/s
> with multiple connections.
> 
> now_ms is not pretected by meter->lock, we may get a negative
> long_delta_ms when another cpu updated meter->used, then:
>     delta_ms = (u32)long_delta_ms;
> which will be a large value.
> 
>     band->bucket += delta_ms * band->rate;
> then we get a wrong band->bucket.
> 
> OpenVswitch userspace datapath has fixed the same issue[1] some
> time ago, and we port the implementation to kernel datapath.
> 
> [1] https://patchwork.ozlabs.org/project/openvswitch/patch/20191025114436.9746-1-i.maximets@ovn.org/
> 
> Fixes: 96fbc13d7e77 ("openvswitch: Add meter infrastructure")
> Signed-off-by: Tao Liu <thomas.liu@ucloud.cn>
> Suggested-by: Ilya Maximets <i.maximets@ovn.org>
> ---
> Changelog:
> v2: just set negative long_delta_ms to zero in case of race for meter lock.
> v1: make now_ms protected by meter lock.
> ---

Thanks!
I didn't test it, but the change looks good to me.

Reviewed-by: Ilya Maximets <i.maximets@ovn.org>

>  net/openvswitch/meter.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/net/openvswitch/meter.c b/net/openvswitch/meter.c
> index 96b524c..896b8f5 100644
> --- a/net/openvswitch/meter.c
> +++ b/net/openvswitch/meter.c
> @@ -611,6 +611,14 @@ bool ovs_meter_execute(struct datapath *dp, struct sk_buff *skb,
>  	spin_lock(&meter->lock);
>  
>  	long_delta_ms = (now_ms - meter->used); /* ms */
> +	if (long_delta_ms < 0) {
> +		/* This condition means that we have several threads fighting
> +		 * for a meter lock, and the one who received the packets a
> +		 * bit later wins. Assuming that all racing threads received
> +		 * packets at the same time to avoid overflow.
> +		 */
> +		long_delta_ms = 0;
> +	}
>  
>  	/* Make sure delta_ms will not be too large, so that bucket will not
>  	 * wrap around below.
>